The upper leg has muscles surrounding the bone. The lower leg, however, doesn't have muscles covering the whole area, leaving the front/inner section of the lower leg to be defined by the bone. Because of the lack of muscle surrounding the knees, their outer form is influenced by the bones and tendons. When the leg is straight, the muscles may appear flatter and less curved than when bent and flexed. When drawing the legs, take into consideration whether the muscles are stretched, flexed or relaxed, all of which influence the legs' appearance.